The present invention relates to an amplitude modulator for mixing multifrequency signals which produces an output of a mixed signal by the use of an alternating current signal. This technique is useful in radar transponder applications such as secondary radar, identification systems and others.
Previously, an amplitude modulator for mixing multifrequency signals has comprised a mixer producing a mixed signal by the inputs of not less than two alternating current signals each having a different frequency than the other and an amplitude modulator.
FIG. 4 shows an embodiment of a conventional amplitude modulator for mixing multifrequency signals. In FIG. 4, the modulator comprises input terminals 6 and 7, a mixer 8, a diode switch 9, a modulation signal generator 10 and an output terminal 11 for modulated mixed signals.
The function of the above-mentioned modulator is explained below. Two alternating current signals e.sub.1 of frequency f.sub.1 and e.sub.2 of frequency f.sub.2, which frequencies differ from each other, are transmitted from input terminals 6 and 7, respectively, to the mixer 8. The mixer 8 generates innumerable side-band and higher harmonics with frequency of n.sub.1 f.sub.1 .+-.n.sub.2 f.sub.2 (where n.sub.1 and n.sub.2 are integers).
The mixed signal, transmitted from mixer 8 to diode switch 9, is modulated into an amplitude modulated signal by a modulation signal e.sub.m of frequency f.sub.m from the modulation signal generator 10. Then, the amplitude modulated signal is transmitted from the output terminal 11. In the device explained here above, the alternating current signals e.sub.1 of frequency f.sub.1 and e.sub.2 of frequency f.sub.2 are applied to the input of mixer 8 and then the mixed signal output e.sub.0 of frequency f.sub.0 from the mixer, is amplitude modulated by diode switch 9. In mathematical terms, e.sub.1, e.sub.2, e.sub.m and e.sub.0 are given by the following equations: ##EQU1## where E.sub.1, E.sub.2, E.sub.m, E.sub.0 : Max. amplitude constant
.omega..sub.1, .omega..sub.2, .omega..sub.m, .omega..sub.0 : Angular velocity (rad/sec) PA1 m.sub.i : Modulation index (0.ltoreq.m.sub.i .ltoreq.1).
Note that the phase angle has been ignored in the above expressions since it is unchanged by the amplitude modulation process.
Signals transmitting information are generally expressed in the form of alternating current. If direct current is used in the expression, it becomes hard to understand the mechanism of information transmission because the components of direct current which do not take part in the information transmission are required to be mathematically expressed.
However, in the case of embodying the information transmission in practice, it is necessary to employ electronic circuits wherein active devices are used. Such active devices need energy supply from the outside to demonstrate the functions thereof. Active devices comprise transistors and diodes which require direct current energy, i.e. direct current bias. Accordingly, in the case of explaining embodiments, the explanation cannot help being made in the form of direct current.
The above description will be clearly understood by the examination of, for example, an alternating current amplifier (see FIG. 5) using a bipolar transistor therein. In FIG. 5, the equation e.sub.o =Ae.sub.i (where A is amplification degree) has a significant meaning. However, when the description of the action of a transistor Q, is required to explain the equation, it is necessary to include the analysis based on direct current in the description. e.sub.i and e.sub.0 are both alternating current, but become such signals that have components of direct current in the circuit. e.sub.i, e.sub.0 and e in FIG. 6 are represented by the following equations. EQU e.sub.i =E.sub.i sin .omega..sub.m t EQU e.sub.0 =E.sub.0 sin .omega..sub.m t EQU e=E.sub.c +E.sub.0 sin .omega..sub.m t
where e is a voltage-applying signal at a collector of the transistor Q.sub.1.
However, in the conventional amplitude modulator for mixing multifrequency signals, there are problems in that (1) both the mixer and amplitude modulator are required, and as a result, it is impossible to reduce the size of the modulator smaller than that of an assembled size of the mixer and amplitude modulator, and (2) when a PIN junction diode, or the like is employed for the diode switch, it requires the mixed signal to be amplitude modulated by transmitting about several tens of milliamperes of current from the circuit for generating the modulation signal to the diode switch.
The present invention overcomes the above-mentioned problems. It is an object of the present invention to simplify and miniaturize the amplitude modulator by combining the mixer and modulation signal generator.